Mining methods and systems including caving to relieve pressure



July 16, 1963 R. F. LOVE ETAL MINING METHODS AND SYSTEMS INCLUDINGGAMING ID RELIEVE PRESSURE 2 Sheets-Sheet l Filed 001.. 3. 1960 July'16, 1963 R. F. LovE ETAL MINING METHODS AND SYSTEMS INCLUDING CAVING TORELIEVE PRESSURE 2 Sheets-Sheet 2 Filed OCT.. I5. 1960 NEEN Wu UnitedStates Patent O 3,097,830 MINING METHODS AND SYSTEMS INCLUDING CAVING TORELIEVE PRESSURE Robert F. Love, Rowayton, Conn., and Thomas S.Bernatis, Rock Springs, Wyo., assignors to FMC Corporation, New York,N.Y., a corporation of Delaware Filed Oct. 3, 1960, Ser. No. 60,054 4Claims. (Cl. 262-1) This invent-ion relates to the mining of trona asfound in Sweetwater County and adjacent counties in southwestern Wyomingand in similar {tron-a deposits found elsewhere.

The trona deposits in Sweetwater County and adjacent counties in Wyomingconsist of a main trona bed having a varying thickness up to about 16feet, averaging more nearly 7 to 13 feet, located at a depth of about1500 to 1800 feet underground and of several thinner beds. Only the maintrona bed is mined. Because of its great depth below the surface and itsrelative hardness, the main Wyoming trona bed is difficult to` mine andthis difficulty is increased by the fact that the main ltrona bed isstronger th-an the formation above yand below it.

The main trona bed lies substantially horizontal. The overlying strataare also horizontal in attitude and are chiefly composed ofunconsolidated shales and fine grained sandstones. The 400 feet ofstrata immediately above the main trona bed are mainly weak, laminatedgrey shales and oil shale, and fan approximately 8 -foot bed of somewhatplastic oil shale lies immedi-ately below the main trona bed.

The comparative strength of the main trona bed and of the overlying andunderlying shale layers in average values is substantially as follows:

Where the trona supporting layer is completely removed as in a miningtunnel or by drawing a pil-lar, neither the roof shale nor the floorshale have suicient strength to sustain the addi-tional burden placedthereon. The roof shale therefore caves and the floor shale heaves intothe mining cavities.

These conditions make conventional mining operations very difricultbecause unlike coal and similar mining operations under a slate,compacted sandstone or other firmly compacted roof, vtrona miningessentially comprises xemoving the only strong layer in the formationand leaving an essentially -weak roof that is incapable of supportingitself over large spans, while the plastic shale immediately below thetrona bed is pushed upward into the mine cavity when the overlyingwei-ght, effecting vertical compression of the plastic shale, isrelieved by the removal of la portion of the trona bed in the miningoperation.

In past Wyoming trona mining operations, it has been the practice tomine essentially rectangular tunnels in the trona formation according tothe conventional roomand-pillar mining system by shear ycutting anddrilling the trona mining face and blasting out the trona.

The trona is mined from secondary entries by the roomand-pillar methodcommonly used in mining coal. Normally in mining the pillars, roomsfifteen feet wide are first driven at right angles to the secondarydevelopment. Several rooms are started at the extreme end of thedevelopment entries with the room closest to the unmined trona bed beingcompleted slightly before the others.

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The rooms are spaced about 50 -to 60 feet apart and 'the resultingpillar between them is extracted. The extraction is normally done bydriving lifts twenty feet wide through the pillars starting from theends of the rooms farthest from the development entry. The lifts areusually driven at right angles to the rooms, and hence parallel to thesecondary development entries, and are separated from the mined out areaby a narrow fender of-trona, fixe feet or less in thickness. When Ealift is completed, [the fender is removed by blasting and theunsupported roof, adjacent the previously mined out and caved tarea,will cave. Thus the mined out areas are caved to relieve the roof Iandfloor pressure adjacent the mined out areas soon 'after the extractionof each section of a pillar is finished.

When one lift has been driven and the blasted, the next parallel lift isstarted, also leaving a five foot fender between it `and the previouslycaved section until it is completed, and this process is repeated untilthe entire pillar has been drawn and the roof thereover caved. The roomsare driven in a direction at right angles away from the entries and thepillars are extracted by lifts at right angles to the rooms or parallelto the entries moving back toward the entires, or on the retreat. Priorto caving, .the roofs of all entries, rooms and lifts are supported byroof bolts, preferably on four foot centers, which are removed as much4as possible before caving. Timbering is used whenever necessary.

Due to the great depth of the trona beds and the overburden weight, theunplanned roof failures and floor heaving in the pillar extraction areashave become so excess-ive as to make the mining opera-tion veryhazardous fand costly, requiring elaborate timbering in some instancesand in other instances requiring the abandonment of mining in specificareas before all the pillars have been drawn.

One of the objects of this invention is to provide a method of stillfur-ther reducing overburden pressures introna mining pillar extractionareas which will reduce the mining hazards land provide safer miningconditions.

Another object of the invention is to provide va methodof protecting thepillar extraction areas from premature or unplanned caving which willreduce the previous cost of timbering .and permit complete pillarextraction before necessity for abandoning :a give area. 1

Other objects and advantages of the invention will appear as thisdescription proceeds. p

We have found that part of the pressure on the pillar extraction areasis due to the superincumbent overburden weight `and that the pillars actas abutments in the working area or room. This excess overburdenpressure can be reduced or dispatched to other more remote areas bydriving a room between the room used for mining the active pillarextraction area and the still unmined trona area and permitting thisfirst room to cave so that the caving extends a substantial distance upinto the overburden. After caving of a room between the activeminingarea and the unmined trona area or pillar, the pillar can be extractedby driving another room very close to the Vcaved room, and lifts canthen be driven to the cave line and the pillar removed. In this manner,vpressure on the active pillar extraction area is reduced and miningconditions rendered less hazardous and mining less costly.

FIG. 2 is a section view on an enlarged scale along'V the line A-A ofFIG. 1; and' 4 FIG. 3 is a section view through a larger area of theformation also substantially on the line A--A of FIG. 1.

Patented July 16, 1963 fenderV As illustrated in FIG. l, a four entrysecondary development panel 1 consisting of entries 1a, 1b, 1c and 1dhas been driven into the trona mining area 2 and communicates at theleft side of the drawing with the main development entries (not shown),through which the men, machines and supplies, etc. are brought into themine and the mined trona removed. A belt conveyor 3 is shown in entry 1con which mined trona brought from the active mining areas 2 is depositedfor conveyance out of the mine. The previously caved panel area is shownat 4 and the freshly caved areas adjacent the active mining operation isshown at 5.

In applying the principles of our invention rooms 6 and `6a are iirstdriven at right angles to the secondary development entry 1d, throughthe bed of trona, to the old caved panel 4, and rooms 7 and 7a areimmediately driven parallel to and between the rst rooms 6 and 6a andthe caved ground 5 from the freshly completed pillar extractionoperation. The rooms 6 and 7 and 6a and 7aare usually left separated byapproximately ten foot pillars 1S and a. While two rooms 6 and 6a andtwo alternate rooms 7 and 7a (one not completed) have been shown tobetter illustrate the application of our invention, it will beunderstood that the mining operation may be moved backward in thedirection of mining indicated by the arrow 12, one pillar at a time, inwhich case only one room 6 will be driven and one alternate room 7, andafter the pillar 8 between the room 7 and the previous mining area 5 hasbeen completely extracted and the area caved, a second room 6a and asecond alternate room 7a is driven and the new pillar 8a is mined.

The pillar 8 between the room 7 and the previous mining operation isshown in the process of being mined. Mining of the pillar 8a is startedwhen room 7a is completed to caved area 4. As the pillars 8, 8a, etc.are being mined by retreat on the entries 1, 1a, 1b and 1c, thesupporting pillars 9 left between the entries are also extracted toremove the trona therein and cave the secondary development panel area1a, 1b, 1c, etc., as well as the room-and-pillar area 8, 8a, etc. Themining of the room pillars 8, 8a, etc. is conducted slightly in advanceof the mining of the entry pillars 9.

Ventilation of the Working areas is carried out through entries 1a, 1b,1c and 1d, and when it is desired to ventilate the caved area 5, asrequired by some mining laws, it is best to leave one row of pillars inplace so that air may be conducted through entry 1a and the crossentries therefrom into the caved area 5. In this manner, fresh air iskept circulating through the cave areas.

The rooms 6, 6a, etc. are left untimbered. These rooms then cave Withina few days after they are driven, with the caving extending sufficientlyfar up into the overburden 10, as indicated at 11, to break the weightof the overburden on the unmined trona layers 12 so that the pressure onthe alternate rooms 7 and 7a and theV pillars 8 and 8a is reducedsuiciently to permit mining of the pillars 8 and 8a without unduehazards and costs. The majority of the roof failures and iloor heavingtakes place in the first rooms 6 and 6a and the second rooms 7 and 7aremain relatively good and pillar extraction between the rooms 7 or 7aand the previously mined area may proceed under much more safe andworkable conditions. Any roof bolts installed in rooms 6 and 6a arerecovered if this can be done safely.

FIG. 2 shows a partial cross section through the mine area andindicates, by the arrows 11, how a portion of the overburden has beencaved into the rooms `6 and 6a to partially break the superincumbenteffect of the overburden over the unmined portions 12 of the trona bed.

As indicated in FIG. 3, which is a diagrammatic crosssection through theformation, the trona layer is approximately 1500 to 1800 feet below theearths surface. This figure illustrates the theory of the invention. Theshale beam roofs of the rooms are weak and will not support muchoverburden weight before cracking and beginning to cave. Part of thesuperincumbent weight of the overburden of the roofs must be supportedby the pillars 8 and 8a on the abutments formed by the vertical walls ofthese pillars, and the amount of superincumbent overburden weight thatthe pillars 8 and 8a must support is a function of the percentage ofextraction of the pillars.

When the first rooms 6 and 6a cave as shown by arrows 13a, it isbelieved that horizontal pressures and deformations towards the cavedrooms cause the extra superincumbent fweight that the pillars 8 and 8amust bear to move back to more solid and confined ground in thedirection of arrow 13. This effects an apparent pressure relief in thevicinity of the deliberately caved rooms 6 or 6a as the horizontalconfining stress, which normally opposes Poissons ratio applied to thevertical weight, can not develop in this region. In this manner, anyvertical Weight is transmitted away from the mining area due to thedeformation of the soft shale beds towards the caved areas.

While we have shown a preferred embodiment of our invention yand setforth certain theories as to why we believe the caving of rooms 6 and 6arelieves the pressure on the :active mining areas, we have in any eventfound this to be the fact regardless of whether the theories advancedherein are correct or not. It will be understood that the invention canbe practiced in other ways than according to the preferred embodimentherein illustrated and that various modifications and changes can bemade in active mining operations within the spirit of our invention andthe scope of the following claims.

We claim:

1. The method of mining trona by the room-and-pillar system and ofrelieving overburden pressure on the pillar extraction area, whichcomprises `driving two substantially parallel rooms spaced approximatelyl0 feet apart between the active pillar extraction area and the unminedtrona area, caving the room nearest the unmined trona area, extractingthe pillar in the pillar extraction area from the parallel room nearestthe pillar extraction area and when the pillars have been extractedcaving the roof over the pillar extraction area and 4advancing the minein the `direction of the unmined trona area by driving two furtherparallel rooms spaced approximately l0 feet apart between the pillararea to be extracted and the unmined trona area, caving the room nearestthe unmined trona area and extracting the pillar in the pillarextraction area from the uncaved room between the caved room and thepillar extraction area and continuing this process in advancing the minein the direction of the unmined trona area.

2. The method of mining trona and relieving overburden pressure in themining operation which comprises driving two adjacent rooms into thetrona bed between the active mining area and the unmined trona area, thesaid rooms being close enough so that upon caving the room adjacent theunmined trona area the overburden pressure will be reduced to avoidunplanned, premature caving in the active mining area, caving the roomadjacent the unmined trona area and, mining the pillars in the activemining area from the uncaved room.

3. The method of mining trona and relieving overburden pressure in themining operation which comprises driving two adjacent rooms into thetrona bed between the active mining area and the unmined trona area, thesaid rooms being close enough so that upon caving the room adjacent theunmined trona area the overburden pressure will be reduced to avoidunplanned, premature caving in the active mining area, caving the roomadjacent the unmined trona area, mining the pillars in the active miningarea from the uncaved room and caving the roof over the pillars in theactive mining area as the pillars are mined.

4. 'I'he method of mining trona by the room and pillar system and ofrelieving overburden pressure on the pillar the pillars in the pillarextraction area from the room yadjacent to the active mining area.

References Cited in the le of this patent UNITED STATES PATENTS Gr-ifthSept. 26, 1911 Bucky Aug. 5, 1958

2. THE METHOD OF MINING TRONA AND RELIEVING OVERBURDEN PRESSURE IN THEMINING OPERATION WHICH COMPRISES DRIVING TWO ADJACENT ROOMS INTO THETRONA BED BETWEEN THE ACTIVE MINING AREA AND THE UNMINED TRONA AREA, THESAID ROOMS BEING CLOSE ENOUGH SO THAT UPON CAVING THE ROOM ADJACENT THEUNMINED TRONA AREA THE OVERBURDEN PRESSURE WILL BE REDUCED TO AVOIDUNPLANNED, PREMATURE CAVING IN THE ACTIVE MINING AREA, CAVING THE ROOMADJACENT THE UNMINED TRONA AREA AND, MINING THE PILLARS IN THE ACTIVEMINING AREA FROM THE UNCAVED ROOM.